1. Field of the Invention
The invention relates to a wobble plate drive for a cluster of axially reciprocating pistons spaced about a shaft axis; and more particularly to such a drive in which the wobble plate tilt angle with respect to the shaft axis may be varied in order to vary the piston displacement. Drives of this type are known for hydraulic pumps and motors, and have been proposed for multi-cylinder thermodynamic engines, particularly of the hot gas type.
Because there is not complete uniformity of terminology in this art, the following definitions are applicable to the specification and appended claims.
Swash plate drive refers specifically to a drive having the bearings for reciprocating the individual pistons in direct contact with a swash plate driving surface that rotates with the shaft. In such a drive the connection between the piston rod or linkage and the swash plate has both a sliding or rolling relative movement with respect to the periphery of the swash plate, and an oscillating pivoted movement with respect to the swash plate. The piston (connecting) rods or linkage are normally constrainted to a purely axial motion. The sliding or rolling movement with respect to the swash plate is then along a surface having circular or elliptical symmetry about a swash plate axis. When the swash plate is tilted so that this axis intersects the shaft axis at a given non-zero angle, the oscillating pivotal movement is about a line which normally lies in a plane perpendicular to the swash plate axis, referred to herein as the driving plane.
A wobble plate drive is a drive having a plate or functionally equivalent member which is coupled through bearings to the bearing surface of a support member whose movement is the same as that of a swash plate driving surface rotating with the shaft. The wobble plate is restrained by a connection to the machine frame so that it does not undergo net rotation about the shaft axis, but merely has a wobbling motion. The wobble plate is connected to the connecting rods through some form of oscillating pivotal bearings whose axes define a driving plane.
2. Description of the Prior Art.
Swash plate drives for hydraulic machines have been in substantial commercial use for a number of years. Because of the high torques and power levels obtainable with hydraulic machines of relatively modest size, extremely rugged shafts and swash plate or wobble plate assemblies have been required. These in turn are mounted in heavy housings or cylinder assemblies. As a result only very small vibration amplitudes are transmitted to the machine mounting points.
Many of the known machines are completely unbalanced insofar as inertia forces caused by piston and piston rod movement and drive mechanism are concerned. For example, U.S. Pat. No. 3,175,363 teaches static balancing of an oblique disc used as a wobble plate, and dynamic balancing for overcoming the effect of sudden reduction in the hydraulic load. Dynamic balancing for the torque about an axis perpendicular to and rotating with the shaft due to the inertia of the pistons moving back and forth is not provided. Also, when the plane is tilted, a greater portion of the hemispheric mass is to one side of the shaft than the other, directly producing a radial imbalance.
The significance of radial imbalance has been appreciated and compensated for. For example, in U.S. Pat. No. 3,292,554 a mechanism is disclosed having a counterweight which moves radially to one side away from the shaft as the hemisphere is moved toward the other side of the shaft. However, the piston inertia forces are again neglected.
More recently, Stirling cycle engines for automotive use have been delivered to Ford Motor Company by N. V. Philips Gloeilampenfabrieken, incorporating a swash plate drive. These engines have a fixed driving plane tilt angle, and are balanced dynamically by choice of the swash plate mass and dimensions. However, because of the use of a plain swash plate, bearing design is complex and may be less easily maintained by average automotive mechanics then drives having standard bearing types.
A variable displacement wobble plate drive in which the inertia forces of the pistons may be counterbalanced is disclosed in copending U.S. application Ser. No. 904,529 filed May 10, 1978, and assigned to the assignee of the instant application. The mechanism disclosed therein overcomes many of the disadvantages of the prior art referred to above, but is still not fully balanced because the restraint mechanism for the wobble plate produces uncompensated pivoting moments.